1. Field of the Invention
The present invention relates to an electroless plating method and an electroless plating apparatus, and more particularly to an electroless plating method and an electroless plating apparatus which, despite using a high-productivity batch processing method, make it possible to stably perform uniform processing of a surface of a substrate, such as a semiconductor wafer.
2. Description of the Related Art
As a three-dimensional packaging technique for electrical connection between semiconductor chips, a method has been proposed in which, as shown in FIG. 1, a microbump 12, provided at a predetermined position on a CPU 10, and a microbump 16, provided at a predetermined position on a memory 14, are used as electrodes, and the microbumps (electrodes) 12, 16 are bonded together.
The microbump 16 provided on the memory 14 is made of, for example, a Cu—Sn alloy. The microbump 12 on the CPU 10 can be formed by forming, e.g., an Ni plated film 20 having a thickness of 2 to 10 μm on a surface of a bump pad 18, e.g., made of Al or Cu, and forming, e.g., an Au plated film 22 having a thickness of 50 to 200 nm on a surface of the Ni plated film 20. Upon the bonding of the microbumps 12 and 16, the Au plated film 22 diffuses into the microbump 16, e.g., made of a Cu—Sn alloy. The Au plated film 22 thus does not contribute to the bonding, but serves to prevent oxidation of the surface of the Ni plated film 20, thereby ensuring the bonding strength.
The microbump 16 provided on the memory 14 is not directly connected to the bump pad 18, e.g., made of Cu, because of damage to, e.g., a low-k material underlying the bump pad 18. Thus, the Ni plated film 20 and the Au plated film 22 are employed as a buffer.
In TSV (through silicon via) interconnection bonding, a method has been proposed which involves bonding a front-surface bump 36, consisting of an Ni plated film 32 formed on the front side of a TSV 30 and an Au plated film 34 formed on the Ni plated film 32, and a back-surface bump 38 formed on the back side of a TSV 30 of another substrate each other, as shown in FIG. 2.
The use of electroless plating, in place of electroplating, is being studied for the formation of the above-described Ni plated films 20, 32 and Au plated films 22, 34. The use of electroless plating, in place of electroplating, is being studied also for the formation of plated films of materials other than Ni and Au, such as Co, Pd, Pt, Cu, Sn, Ag, Rh, Ru etc., or a composite material thereof.
Al and Cu are commonly used as the metal (bump pad 18) underlying the microbump 12, shown in FIG. 1. Al and Cu are not catalytic metals such as Fe, Co, Ni, Pd, Pt, etc. Therefore, when the underlying metal is Al or Cu, a zincate treatment for the Al surface or a Pd catalyst application treatment (or initial application of electric current) for the Cu surface is generally carried out as a pre-plating treatment. In the zincate treatment of the Al surface, zinc is generally applied to the Al surface by displacement plating. Zinc itself acts as a catalyst poison and inhibits a catalytic action. It is therefore necessary to control the amount of zinc plated (displaced) at an appropriate level in the zincate treatment. In the subsequent electroless plating, the zinc is replaced with a catalytic metal with which electroless plating is possible.
As described above, a pre-plating treatment (activation treatment) of an Al surface or a Cu surface is effected by displacement plating with, e.g., a catalytic metal or zinc. For example, a Pd catalyst application treatment can be carried out, e.g., by immersing a substrate in a sulfuric acid-based solution containing palladium sulfate. A zincate treatment can be carried out by immersing a substrate in a sodium hydroxide-based solution containing zinc oxide. Prior to a pre-plating (activation) treatment of a substrate surface, the substrate surface is generally cleaned, e.g., with nitric acid or citric acid to remove an oxide film or contaminants from the substrate surface, and then cleaned with water. The pre-plating treatment is carried out subsequent to the water cleaning and without predrying of the substrate surface. Electroless plating of the substrate surface is carried out subsequent to the cleaning with water of the substrate surface after the pre-plating treatment and without predrying of the substrate surface. Thus, the sequence of electroless plating processing steps is carried out successively without drying of the substrate surface. This is because if the substrate surface is dried after the pre-cleaning or the pre-plating treatment, then an oxide film will be formed on the substrate surface, leading to poor plating.
Pre-plating treatments can cause various problems such as: damage to an underlying material, poor adhesion between a displacement plated film and an underlying metal, roughening of a plated film, etc. in the case of a patterned wafer; and poor uniformity of the surface morphology, leading to poor appearance, etc. in the case of an unpatterned wafer.
In a pre-plating treatment (activation treatment) for electroless plating, it is important to perform displacement plating with Zn or Pd densely and uniformly. The amount of displacement plating and the amount of surface etching are greatly influenced by the concentration of an etching agent contained in a treatment solution, for example, the concentration of NaOH in a pre-plating treatment (zincate treatment) of an Al surface with a sodium hydroxide-based solution containing zinc oxide, or the concentration of sulfuric acid in a pre-plating treatment (Pd catalyst application treatment) of a Cu surface with a sulfuric acid-based solution containing palladium sulfate. In some cases, the amount of displacement plating and the amount of surface etching are determined in a few seconds.
Electroless plating apparatuses can be classified roughly into electroless processing apparatuses using a single-substrate processing method in which substrates are processed in a one-by-one manner, and electroless plating apparatuses using a batch processing method in which a plurality of substrates are simultaneously held and processed. The plating rate in electroless plating is generally 1 to 20 μm/s, which is significantly lower than that of electroplating. Therefore, when employing electroless plating for processing that requires a lot of time, such as the formation of bumps, the use of a batch processing method rather than a single-substrate processing method is generally preferred.
Electroless plating apparatuses of the batch processing type have the advantage that with the same footprint, the throughput is much higher as compared to the single-substrate processing type. On the other hand, in an electroless plating apparatuses of the batch processing type, a plurality of substrates, arranged parallel to each other and each held in a vertical position, are simultaneously immersed and processed in a processing liquid, such as a pre-plating treatment solution or a plating solution. Therefore, a metal in a plating solution, for example, is consumed generally in a large amount during one batch processing. Further, a large amount of a processing liquid, such as a liquid chemical or pure water, will adhere to substrates and will be brought out of the processing tank. In addition, it is necessary to frequently perform flushing of, e.g., a plating solution circulation line to remove extraneous matter from the interior of the line, e.g., with an etching liquid. Furthermore, compared to the single-substrate processing type, an electroless plating apparatuses of the batch processing type is generally inferior in the processing performance (the uniformity of the thickness and the quality of a plated film) that is dependent on the time it takes to fully immerse a substrate, from the upper end to the lower end, in a processing liquid, the direction of the flow of a processing liquid, the uniformity of the temperature distribution in a processing liquid, etc.
In electroless plating using a batch processing method, it is common practice to replenish a plating solution with a metal by the following replenishing methods: estimated metal replenishment based on the consumption of the metal, calculated from the plating area and the plating time; and metal replenishment based on the results of periodic analysis of the plating solution. For example, estimated replenishment of metal ions may be performed per batch and, in addition, replenishment of metal ions based on an analysis of the plating solution may be performed every few days. The full replacement of the plating solution may be made when the amount of the metal deposited reaches a certain value.
The applicant has proposed a substrate processing apparatus of the batch processing type, including a substrate holder for holding a plurality of substrates and immersing the substrates in a processing liquid in a processing tank. The substrate holder, holding the substrates, is rotated in the processing liquid in the processing tank (see patent document 1). A substrate processing apparatus of the batch processing type has also been proposed which includes a carrier stage, a horizontal transfer robot, a posture conversion mechanism, a pusher, a transport mechanism and a substrate processing section, and which is configured to transport and process a plurality of substrates in parallel (see patent document 2). Further, an electroless plating tank has been proposed which is capable of creating a uniform flow of a plating solution over a surface of a flat plate-like plating object. The plating tank is provided with a pair of impellers disposed on both sides of the plating object and which rotate in the same direction to create a flow of the plating solution between the plating object and a guide plate. The direction of rotation of the impellers is periodically reversed (see patent document 3).